function baz() {
    console.log('baz')
    bar()
}

function bar() {
    console.log('bar')
    foo()
}

function foo() {
    console.log('foo')
}

baz()


{
    // let a = 10; // 外层访问不到 ReferenceError
    var a  = 100; // 外层访问的到
    console.log(a)
}

console.log(a) // 块级里面用let const; ReferenceError

